Steering device for wheel loader

ABSTRACT

A steering device for a wheel loader includes a handle for steering, a lever for steering, a steering cylinder, a steering valve that controls a direction of operating fluid to the steering cylinder, and a pump that supplies operating fluid to the steering cylinder via the steering valve. The steering device further includes a handle control valve that controls the supply direction of the operating fluid according the handle, a lever electromagnetic control valve that controls the supply direction of operating fluid according to the lever, and a handle prioritization valve that shuts off a pilot hydraulic pressure supplied from the lever electromagnetic control valve to the steering valve according to a pressure produced by operating fluid flowing through the handle control valve when the handle is operated. The handle prioritization valve includes a switching valve and a priority control valve switchable between a handle stage and a lever stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/122,405 filed on Nov. 26, 2013. U.S. patent application Ser.No. 14/122,405 is a U.S. National stage application of InternationalApplication No. PCT/JP2012/060541, filed on Apr. 19, 2012, which claimspriority under 35 U.S.C. §119(a) to Japanese Patent Application No.2012-076093, filed in Japan on Mar. 29, 2012. The entire contents ofU.S. patent application Ser. No. 14/122,405 and Japanese PatentApplication No. 2012-076093 are hereby incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a steering device and in particular toa steering device for a wheel loader in which a front frame and a rearframe are pivotably coupled in the horizontal direction.

2. Background Information

A wheel loader is provided with a handle for performing a steeringoperation when traveling at a high speed on a public road or at a worksite. A lever such as a joystick is also provided to allow for effectivesteering with simple operations at a work site where steering operationsfor short distances are repeated.

There is a possibility that an unintentional steering operation may beperformed when the driver mistakenly touches the lever while operatingthe handle on a wheel loader in which both handle and lever operationsmay be used for steering as described above.

Accordingly, a control device for a material handling machine describedin Japanese Laid-open Patent Publication No. 2005-535487 is configuredso that a handle operation is prioritized when the handle and the leverare operated at the same time. Specifically, the device described inJapanese Laid-open Patent Publication No. 2005-535487 is provided with asequence valve along with a control valve for a lever operation. Whenthe handle is operated, the sequence valve is actuated by operatingfluid fed by a control valve for the handle operation, and the supply ofthe operating fluid from the control valve for the lever operation to asteering cylinder is blocked.

SUMMARY

The control device in Japanese Laid-open Patent Publication No.2005-535487 is provided with two electromagnetic control valves and asequence valve as means for prioritizing the handle operation, thevalves being provided separately from the control valve for the handleoperation and the control valve for the lever operation. Thus, theconfiguration for prioritizing the handle operation is complex.

Moreover, in the control device of Japanese Laid-open Patent PublicationNo. 2005-535487, the sequence valve and the electromagnetic controlvalves are actuated by operating fluid flowing through the valve for thehandle operation and the pressure at both ends of the control valve forthe lever operation is the same, and as a result the supply of operatingfluid from the control valve for the lever operation to the steeringcylinder is stopped.

However, with this type of configuration, the action of the sequencevalve becomes unstable due to the amount of operating fluid flowingthrough the control valve for the handle operation, and the handleoperation cannot be continuously and reliably prioritized.

An object of the present invention is to achieve a mechanism forprioritizing handle operations with a simple configuration and toactuate the mechanism in a stable manner in a wheel loader in whichsteering can be performed by a handle operation and a lever operation.

A steering device for a wheel loader according to a first aspect of thepresent invention includes a handle that is operated by rotating forsteering, a lever that is operated by tilting for steering, a steeringcylinder for pivoting a front frame in relation to a rear frame byexpansion and contraction of the steering cylinder, a steering valve, apump, a handle control valve, a lever electromagnetic control valve, anda handle prioritization valve. The steering valve uses pilot hydraulicpressure to control the supply direction of operating fluid to thesteering cylinder. The pump supplies operating fluid to the steeringcylinder via the steering valve. The handle control valve supplies pilothydraulic pressure for controlling the supply direction to the steeringvalve according to a rotation operation of the handle. The lever controlvalve supplies pilot hydraulic pressure for controlling the supplydirection to the steering valve according to a tilting operation of thelever. The handle prioritization valve shuts off the pilot hydraulicpressure supplied from the lever electromagnetic control valve to thesteering valve according to a differential pressure or an absolutepressure in a hydraulic circuit produced due to the operating fluidflowing through the handle control valve when the handle is operated.

When the handle is operated in this device, the pilot hydraulic pressurefor controlling the flow direction of the operating fluid is supplied tothe steering valve via the handle control valve. Moreover, when thelever is operated, the pilot hydraulic pressure for controlling the flowdirection of the operating fluid is supplied to the steering valve viathe lever control valve. Since the operating fluid flows through thehandle control valve when the handle is operated, the handleprioritization valve is actuated due to the differential pressure orabsolute pressure in the hydraulic circuit through which the operatingfluid flows. Due to the actuation of the handle prioritization valve,the pilot hydraulic pressure supplied from the lever electromagneticcontrol valve to the steering valve is shut off.

When the handle is operated, the pilot hydraulic pressure to be suppliedfrom the lever electromagnetic control valve to the steering valve isshut off and the handle operation is prioritized even if the lever isoperated while the handle is being operated. Therefore, an unintentionalsteering operation can be prevented even if the driver mistakenlytouches the lever while the handle is being operated. Moreover, sincethe pilot hydraulic pressure from the lever electromagnetic controlvalve is shut off by the handle prioritization valve that is actuateddue to the differential pressure or absolute pressure in the hydrauliccircuit produced by the handle operation, the circuit configuration issimplified and unstable operation of the handle prioritization valve canbe prevented.

In a steering device for a wheel loader according to a second aspect ofthe present invention, the handle prioritization valve in the device ofthe first aspect has a first throttle provided in a circuit forsupplying operating fluid to the handle control valve, and aprioritization valve body. The prioritization valve body has acommunication stage for supplying pilot hydraulic pressure from thelever electromagnetic control valve to the steering valve, and a shutoffstage for shutting off the supply of the pilot hydraulic pressure to thesteering valve, and when a differential pressure is produced on eitherside of the first throttle, the communication stage is switched to theshutoff stage.

When the handle is operated in this device, the operating fluid flows tothe handle control valve. A differential pressure is produced on eitherside of the first throttle due to the flow of the operating fluid. Theprioritization valve body switches to the shutoff stage due to thedifferential pressure. As a result, the pilot hydraulic pressuresupplied from the lever electromagnetic control valve to the steeringvalve is shut off.

Since the differential pressure is produced by the first throttle andthe prioritization valve body is actuated, the handle operation can beprioritized in a stable manner with a simple mechanism.

In a steering device for a wheel loader according to a third aspect ofthe present invention, the handle prioritization valve of the deviceaccording to the second aspect further includes a second throttle thathas a smaller aperture diameter than that of the first throttle and thatis provided in a circuit for supplying operating fluid to the leverelectromagnetic control valve.

The handle operation is preferably prioritized even when theprioritization valve body does not operate in a normal way.

The second throttle having a smaller aperture diameter than that of thefirst throttle on the handle control valve side is provided in thecircuit for supplying operating fluid to the lever electromagneticcontrol valve.

Since the aperture diameter of the first throttle is relatively largeand the aperture diameter of the second throttle is relatively small,more operating fluid is supplied to the handle control valve than to thelever electromagnetic control valve and the handle operation isprioritized even if the prioritization valve body does not operate in anormal way.

A steering device for a wheel loader according to a fourth aspect of thepresent invention further includes a check valve provided parallel tothe first throttle in the device of the second or third aspect, thecheck valve being opened to supply operating fluid to the handle controlvalve when the differential pressure on either side of the firstthrottle is equal to or greater than a certain pressure, and stopping areverse flow of operating fluid from the handle control valve.

The check valve is provided parallel to the first throttle. Therefore,the check valve opens when the differential pressure required foractuating the prioritization valve body is exceeded so that an increasein pressure loss during a large flow rate can be suppressed. Moreover,even if the first throttle is temporarily blocked, operating fluid canbe supplied to the handle control valve via the check valve when thecircuit pressure is equal to or greater than a certain pressure.

A steering device for a wheel loader according to a fifth aspect of thepresent invention further includes a left pilot oil passage and a rightpilot oil passage provided between the handle control valve and thesteering valve in the device of the first aspect. The handleprioritization valve includes a priority control valve and a switchingvalve. The priority control valve is disposed between the steering valveand the handle control valve and the lever electromagnetic controlvalve, and includes a handle stage that allows communication between thehandle control valve and the steering valve and shuts off communicationbetween the lever electromagnetic control valve and the steering valve,and a lever stage that shuts off communication between the handlecontrol valve and the steering valve and allows communication betweenthe lever electromagnetic control valve and the steering valve. Theswitching valve switches the priority control valve to the handle stagewhen a differential pressure is produced between the left pilot oilpassage and the right pilot oil passage.

When the handle is operated in this device, the operating fluid flows tothe handle control valve. As a result, a differential pressure isproduced between the left pilot oil passage and the right pilot oilpassage provided on the output side of the handle control valve. Thepriority control valve is switched to the handle stage by the switchingvalve when this differential pressure is produced.

In this case, the handle operation is accurately prioritized in a stablemanner when the handle is operated.

A steering device for a wheel loader according to a sixth aspect of thepresent invention further includes a left lever pilot oil passage and aright lever pilot oil passage provided between the lever control valveand the priority control valve in the device of the fifth aspect. Theswitching valve switches the priority control valve to the lever stagewhen no differential pressure exists between the left pilot oil passageand the right pilot oil passage and when a differential pressure isproduced between the left lever pilot oil passage and the right leverpilot oil passage.

When the handle is not operated in this device, the operating fluid doesnot flow to the handle control valve and no differential pressure isproduced between the left and right pilot passages. Conversely, when thelever is operated, a differential pressure is produced between the leftand right lever pilot passages. In this way, the priority control valveis switched to the lever stage by the switching valve when adifferential pressure is produced between the left and right pilotpassages on the lever side.

When a lever operation is performed in this case, a steering operationcan be performed accurately with a lever operation.

A steering device for a wheel loader according to a seventh aspect ofthe present invention further includes a left pilot oil passage and aright pilot oil passage provided between the handle control valve andthe steering valve in the device of the first aspect. The handleprioritization valve includes a priority control valve and a switchingvalve. The priority control valve is disposed between the steering valveand the handle control valve and the lever electromagnetic controlvalve, and includes a handle stage that allows communication between thehandle control valve and the steering valve and shuts off communicationbetween the lever electromagnetic control valve and the steering valve,and a lever stage that shuts off communication between the handlecontrol valve and the steering valve and allows communication betweenthe lever electromagnetic control valve and the steering valve. Theswitching valve switches the priority control valve to the handle stagewhen operating fluid flows to the handle control valve, and switches thepriority control valve to the lever stage when operating fluid flowsonly to the lever electromagnetic control valve.

When the handle is operated in this device, the operating fluid flowsthrough the handle control valve to the left pilot oil passage or theright pilot oil passage. The priority control valve is switched to thehandle stage by the switching valve upon receiving the flow of theoperating fluid. When only the lever is operated and the operating fluidflows only to the lever electromagnetic control valve, the prioritycontrol valve is switched to the lever stage by the switching valve.

Switching can be performed accurately and in a stable manner inaccordance with the operations.

A steering device for a wheel loader according to an eighth aspect ofthe present invention further includes a left pilot oil passage and aright pilot oil passage provided between the handle control valve andthe steering valve, and a return oil passage for guiding return oil fromthe steering valve to a tank via the handle control valve, in the deviceof the first aspect. Moreover, output ports of the lever electromagneticcontrol valve are connected to the left and right pilot oil passages.The handle prioritization valve further includes a throttle and aprioritization valve body. The throttle is provided between the handlecontrol valve and the tank in the return oil passage. The prioritizationvalve body is provided between the output ports of the leverelectromagnetic control valve and the left and right pilot oil passages,and includes a communication stage for allowing communication betweenthe output ports of the lever electromagnetic control valve and the leftand right pilot oil passages, and a shutoff stage for shutting offcommunication between the output ports of the lever electromagneticcontrol valve and the left and right pilot oil passages. Theprioritization valve body is switched to the shutoff stage when adifferential pressure is produced on either side of the throttle, and isswitched to the communication stage when no differential pressure isproduced on either side of the throttle.

When the handle is operated in this device, the operating fluid isreturned from the steering valve to the tank via the handle controlvalve. Therefore, a differential pressure is produced on either side ofthe throttle provided in the return oil passage. In this case, theprioritization valve body is switched to the shutoff stage. Therefore,operating fluid from the lever electromagnetic control valve is notsupplied to the steering valve. Specifically, the handle operation isprioritized. Moreover, when the handle is not operated and the lever isoperated, no differential pressure is produced on either side of thethrottle. In this case, the prioritization valve body is switched to thecommunication stage. Therefore, the operating fluid from the leverelectromagnetic control valve is supplied to the left pilot oil passageor the right pilot oil passage via the steering valve and a steeringoperation using the lever is possible.

A steering device for a wheel loader according to a ninth aspect of thepresent invention further includes a check valve provided parallel tothe throttle in the device of the eighth aspect, the check valve openingto return operating fluid to the tank when the differential pressure oneither side of the throttle is equal to or greater than a certainpressure.

The check valve opens when the differential pressure required foractuating the prioritization valve body is exceeded so that an increasein pressure loss during a large flow rate can be suppressed. Moreover,operating fluid is returned from the handle control valve side to thetank side via the check valve when the throttle is clogged. As a result,disablement of a handle operation is prevented.

The wheel loader in which steering can be performed by a handleoperation and a lever operation in the present invention as describedabove realizes a mechanism for prioritizing the handle operation with asimple configuration, and furthermore the prioritization of the handleoperation can be actuated in a stable manner.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an external appearance of a wheel loader.

FIG. 2 illustrates a system configuration of a steering device accordingto a first embodiment of the present invention.

FIG. 3 describes a flow of operating fluid when a handle operation isconducted according to the first embodiment of the present invention.

FIG. 4 describes a flow of operating fluid when a joystick operation isconducted according to the first embodiment of the present invention.

FIG. 5 illustrates a system configuration of a steering device accordingto a second embodiment of the present invention.

FIG. 6 describes a flow of operating fluid when a handle operation isconducted according to the second embodiment of the present invention.

FIG. 7 describes a flow of operating fluid when a joystick operation isconducted according to the second embodiment of the present invention.

FIG. 8 illustrates a system configuration of a steering device accordingto a third embodiment of the present invention.

FIG. 9 describes a flow of operating fluid when a handle operation isconducted according to the third embodiment of the present invention.

FIG. 10 describes a flow of operating fluid when a joystick operation isconducted according to the third embodiment of the present invention.

FIG. 11 illustrates a system configuration of a steering deviceaccording to a fourth embodiment of the present invention.

FIG. 12 describes a flow of operating fluid when a handle operation isconducted according to the fourth embodiment of the present invention.

FIG. 13 describes a flow of operating fluid when a joystick operation isconducted according to the fourth embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENT(S) First Embodiment OverallConfiguration of Wheel Loader

FIG. 1 illustrates an overall configuration of a wheel loader 1 on whichis mounted a steering device according to the first embodiment of thepresent invention. FIG. 1 is a side view of the wheel loader 1.

The wheel loader 1 includes a vehicle frame 2, working equipment 3, apair of front tires 4, an operating cabin, an engine room 6, and a pairof rear tires 7.

The vehicle frame 2 is a so-called articulated construction and includesa front frame 11, a rear frame 12, and a coupling part 13. The frontframe 11 is disposed in front of the rear frame 12 and is coupled to therear frame 12 by the coupling part 13 in a manner that allows pivotingin the left and right directions.

The working equipment 3 includes a lift arm 15 and a bucket 16. The liftarm 15 and the bucket 16 are driven by pressure oil supplied by ahydraulic pump which is not illustrated. A proximal end of the lift arm15 is pivotably supported on the front frame 11. The bucket 16 ispivotably attached to a distal end part of the lift arm 15.

The operating cabin 5 is disposed on the rear frame 12. A handle and ajoystick for performing steering operations, an operating member foroperating the working equipment, and various display devices aredisposed inside the operating cabin 5.

(Steering Device 20)

FIG. 2 illustrates a configuration of a steering device 20. The steeringdevice 20 is a device for turning the vehicle by pivoting the frontframe 11 with respect to the rear frame 12.

The steering device 20 includes a handle 21 that is rotated, a joystick22 that is tilted, and left and right steering cylinders 23L and 23R.The handle 21 and the joystick 22 are members for steering. By operatingthe members 21 and 22, the left and right steering cylinders 23L and 23Rextend and contract to allow the front frame 11 to pivot with respect tothe rear frame 12.

The steering device 20 further includes a steering valve 25, an orbitroll 26 as a handle control valve, a lever electromagnetic control valve27, and a handle prioritization valve 28.

(Steering Valve 25)

The steering valve 25 is a member for supplying operating fluid from apump 30 to either of the left and right steering cylinders 23L and 23Rin response to pilot hydraulic pressure from the orbit roll 26 or thelever electromagnetic control valve 27.

Specifically, a directional control valve 25 a that configures thesteering valve 25 is switched a left turning stage L when operatingfluid is supplied to a left pilot port, and is switched to a rightturning stage R when operating fluid is supplied to a right pilot port.When the directional control valve 25 a is switched to the left turningstage L, operating fluid from the pump 30 is supplied to a bottom-sideoil chamber of the left steering cylinder 23L and to a rod-side oilchamber of the right steering cylinder 23R, and the operating fluidinside the bottom-side oil chamber of the right steering cylinder 23Rand inside the rod-side oil chamber of the left steering cylinder 23L isreturned to a tank 31 via a drain oil passage Pd. Conversely, when thedirectional control valve 25 a is switched to the right turning stage R,operating fluid from the pump 30 is supplied to a bottom-side oilchamber of the right steering cylinder 23R and to a rod-side oil chamberof the left steering cylinder 23L, and the operating fluid inside thebottom-side oil chamber of the left steering cylinder 23L and inside therod-side oil chamber of the right steering cylinder 23R is returned tothe tank 31 via the drain oil passage Pd. If operating fluid is notsupplied to the left and right pilot ports, the directional controlvalve 25 a is switched to a neutral stage N and the cylinders of theleft and right steering cylinders 23L and 23R are shut off from the tank31.

Both of the pilot ports of the directional control valve 25 acommunicate via a throttle 25 b. If operating fluid is supplied to oneof the pilot ports, operating fluid in the oil passage of the otherpilot port side is returned to the tank 31 via the orbit roll 26 and thedrain oil passage Pd.

While details are omitted, the steering valve 25 includes, in additionto the above directional control valve 25 a, an unloading valve, arelief valve, and a safety valve and the like.

(Orbit Roll 26)

An input side P port of the orbit roll 26 is connected to a pump (notillustrated) for pilot hydraulic pressure, such as a fan pump, via afirst input oil passage P01. The orbit roll 26 is a member fordischarging operating fluid proportionate to a handle rotating amountfrom an L port when the handle 21 is rotated to the left and from an Rport when the handle 21 is rotated to the right. A left pilot oilpassage PL is connected to the output side of the L port, and a rightpilot oil passage PR is connected to the output side of the R port.

The left pilot oil passage PL is connected to the left pilot port of thedirectional control valve 25 a that configures the steering valve 25.The right pilot oil passage PR is connected to the right pilot port ofthe directional control valve 25 a. Stop valves 34 and 35 are providedrespectfully in the left and right pilot passages PL and PR. The stopvalves 34 and 35 are members for forcefully stopping the supply of pilothydraulic pressure to the steering valve 25.

A T port on the input side of the orbit roll 26 is connected to the tank31 via the drain oil passage Pd. As described above, when operatingfluid is supplied to either of the pilot ports of the directionalcontrol valve 25 a, the passage on the other pilot port side isconnected to the drain oil passage Pd via the T port.

(Lever Electromagnetic Control Valve 27)

A P port on the input side of the lever electromagnetic control valve 27is connected, via a second input oil passage P02, to the pump connectedto a first input oil passage P01. The lever electromagnetic controlvalve 27 is a member for discharging operating fluid input from the Pport to the L port or the R port on the output side in response to anoperation of the joystick 22. A left lever pilot oil passage PL1 isconnected to the L port, and the left lever pilot oil passage PL1 isconnected to the left pilot oil passage PL. A right lever pilot oilpassage PR1 is connected to the R port, and the right lever pilot oilpassage PR1 is connected to the right pilot oil passage PR. The handleprioritization valve 28 is provided on the left lever pilot oil passagePL1 and the right lever pilot oil passage PR1.

As described above, operating fluid from the lever electromagneticcontrol valve 27 is supplied to the pilot ports of the steering valve 25via the handle prioritization valve 28 and the left and right pilot oilpassages PL and PR.

(Handle Prioritization Valve 28)

The handle prioritization valve 28 includes a prioritization valve body40, a first throttle 41, a check valve 42, and a second throttle 43.

The prioritization valve body 40 is provided in the left and right leverpilot oil passages PL1 and PR1 on the output side of the leverelectromagnetic control valve 27. The prioritization valve body 40 isswitched between a communication stage St and a shutoff stage Sb due topilot hydraulic pressure on either side of the first throttle 41. Thefirst throttle 41 is provided in the first input oil passage P01. Thecheck valve 42 is provided parallel to the first throttle 41 andsupplies operating fluid from the first input oil passage P01 to theorbit roll 26 when the differential pressure on either side of the firstthrottle 41 is equal to or greater than a certain pressure. The secondthrottle 43 is provided in the second input oil passage P02 and is setso that the aperture diameter is less than that of the first throttle41. A filter 44 is provided on the input side of the second throttle 43.

The prioritization valve body 40 is switched to the communication stageSt by a spring when pilot hydraulic pressure does not act on theprioritization valve body 40. Conversely, the prioritization valve body40 is switched to the shutoff stage Sb when operating fluid flows to thefirst throttle 41 and the hydraulic pressure on the input side of thefirst throttle 41 becomes higher than the output side of the firstthrottle 41.

(Steering Operation)

The following is an explanation of a steering device operation whenturning to the left. An operation when turning to the right is basicallythe same except that the oil passages through which the pilot andsteering operating fluids flow are different.

(Handle 21 Operation)

FIG. 3 illustrates the flow of steering operating fluid (solid lines)and pilot operating fluid (dashed lines) when the handle is operated fora turn to the left.

When the handle 21 is operated, pilot operating fluid is input from thefirst input oil passage P01 to the orbit roll 26. Specifically, theoperating fluid passes through the first throttle 41. As a result, thehydraulic pressure on the input side of the first throttle 41 is higherthan the hydraulic pressure on the output side. As a result, theprioritization valve body 40 is switched to the shutoff stage Sb.

In the condition described above, the operating fluid input from thefirst input oil passage P01 is output from the orbit roll 26 to the leftpilot oil passage PL. The operating fluid is input to the left pilotport of the directional control valve 25 a of the steering valve 25 viathe stop valve 35. The directional control valve 25 a is switched to theleft turning stage L due to the pilot hydraulic pressure.

At this time, the operating fluid in the right pilot oil passage PR thatcommunicates with the right pilot port of the directional control valve25 a is returned to the tank 31 via the orbit roll 26 and the drain oilpassage Pd.

When the directional control valve 25 a of the steering valve 25 isswitched to the left turning stage L, operating fluid from the pump 30is supplied to the bottom-side oil chamber of the left steering cylinder23L and to the rod-side oil chamber of the right steering cylinder 23Rvia the steering valve 25.

The operating fluid inside the bottom-side oil chamber of the rightsteering cylinder 23R and inside the rod-side oil chamber of the leftsteering cylinder 23L is returned to the tank 31 via the steering valve25.

When the flow rate inside the first input oil passage P01 becomes large,the differential pressure on either side of the first throttle 41 is nolonger greater than the required differential pressure and pressure lossincreases. In this case, the check valve 42 opens and the differentialpressure increases, that is, an increase in pressure loss is suppressed.The hydraulic pressure of the first input oil passage P01 increases whenthe first throttle 41 becomes clogged during a handle operation. Whenthe hydraulic pressure equal or exceeds a certain hydraulic pressure,the check valve 42 opens and operating fluid is supplied to the orbitroll 26.

(Joystick 22 Operation)

FIG. 4 illustrates a case in which only the joystick 22 is operated andthe handle 21 is not operated. The flow of steering operating fluid isillustrated with solid lines, and the flow of the pilot operating fluidis illustrated with dashed lines in FIG. 4.

When the handle 21 is not operated, operating fluid does not flow to thefirst throttle 41. Therefore, no differential pressure is produced oneither side of the first throttle 41. In this case, the prioritizationvalve body is switched to the communication stage St.

In the condition described above, operating fluid input from a secondinput oil passage P02 is output from the joystick 22 to the left leverpilot oil passage PL1. The operating fluid is supplied to the left pilotoil passage PL and, in the same way as described above for the handleoperation, is input to the left pilot port of the directional controlvalve 25 a of the steering valve 25. Therefore, the directional controlvalve 25 a is switched to the left turning stage L. The pilot oil in theright pilot oil passage PR is returned from the tank port T of the leverelectromagnetic control valve 27 to the tank 31 via the drain oilpassage Pd.

The flow of the steering operating fluid is exactly the same as for thehandle operation.

(When the Handle 21 and the Joystick 22 are Operated)

The joystick 22 may be operated when the operator mistakenly touches thejoystick 22 while operating the handle 21. In this case, the handleoperation is prioritized.

Specifically, when the handle 21 is operated, a differential pressure isproduced on either side of the first throttle 41 and the prioritizationvalve body 40 is switched to the shutoff stage Sb as described above(see FIG. 3). Therefore, even if the joystick 22 is operated during theabove condition, the operating fluid from the lever electromagneticcontrol valve 27 is shut off by the prioritization valve body 40.

(If Prioritization Valve Body 40 is Faulty)

If the prioritization valve body 40 has a fault, the prioritizationvalve body 40 may be switched to the shutoff stage Sb even when thehandle 21 is being operated.

Operating fluid is supplied to both the orbit roll 26 and the leverelectromagnetic control valve 27 under such a state. However, since theaperture diameter of the first throttle 41 is greater than that of thesecond throttle 43, the maximum flow rate to the P port of the orbitroll 26 is greater than the maximum flow rate to the P port of the leverelectromagnetic control valve 27. As a result, the operation of thehandle 21 is prioritized.

(Operation of Stop Valves 34, 35)

The stop valve 35 is mechanically shifted to a stage having a checkvalve when the wheel loader l is pivoted to the maximum limit forturning left. As a result, operating fluid from the orbit roll 26 is notsupplied to the directional control valve 25 a. Conversely, the left andright pilot passages PL and PR both become “0” since the operating fluidinside the right pilot oil passage is returned to the tank 31. As aresult, the wheel loader 1 is able to maintain being steered to the leftat the maximum limit. Steering to the right includes the above actionsin reverse.

(Effects of First Embodiment)

Handle operations are prioritized when the handle 21 is operated.Therefore, an unintentional steering operation can be prevented even ifthe operator mistakenly touches the joystick 22 while the handle isbeing operated.

(2) The circuit configuration is simple and operations are conducted ina stable manner since the prioritization valve body 40 is switched tothe shutoff stage Sb due to a differential pressure on either side ofthe first throttle 41 produced due to a handle operation.

(3) The aperture diameter of the first throttle 41 on the handleoperation side is set to be greater than the aperture diameter of thesecond throttle 43 on the joystick operation side. Therefore, even ifthe prioritization valve body 40 does not operate normally, moreoperating fluid is supplied to the orbit roll 26 and the handleoperation is prioritized.

(4) The check valve 42 is provided parallel to the first throttle 41 andoperating fluid is supplied to the orbit roll 26 via the check valve 42when the differential pressure on either side of the first throttle 41equals or exceeds a certain pressure. Therefore, even if a large amountof operating fluid is supplied to the first throttle 41, an increase inpressure loss is suppressed since the operating fluid flows via thecheck valve 42. Moreover, handle operations can still be effective evenif the first throttle 41 is clogged.

Second Embodiment

FIG. 5 illustrates a steering device according to a second embodiment ofthe present invention. Only a handle prioritization valve 50 in thesteering device is different from the first embodiment and otherconfigurations are the same as those in the first embodiment. Thefollowing is an explanation of the handle prioritization valve 50 thatis different from the first embodiment.

(Handle Prioritization Valve 50)

The handle prioritization valve 50 includes a priority control valve 51provided in the left and right pilot oil passages PL and PR, and aswitching valve 52 for switching the priority control valve 51.

(Priority Control Valve 51)

The left and right output ports of the orbit roll 26 are connected toinput ports of the priority control valve 51 via the left and rightpilot oil passages PL and PR. The left and right output ports of thelever electromagnetic control valve 27 are connected to other inputports of the priority control valve 51. The input ports of the prioritycontrol valve 51 and the left and right output ports of the leverelectromagnetic control valve 27 are connected by the left lever pilotoil passage PL1 and the right lever pilot oil passage PR1. The prioritycontrol valve 51 is able to switch between a handle stage Sh and a leverstage S1 based on pilot hydraulic pressure from the switching valve 52.Specifically, the priority control valve is switched to the handle stageSh when pilot hydraulic pressure is not supplied, and switched to thelever stage S1 when pilot hydraulic pressure is supplied from theswitching valve 52.

The handle stage Sh allows communication between the orbit roll 26 andthe steering valve 25 and shuts off communication between the leverelectromagnetic control valve 27 and the steering valve 25. The handlestage S1 shuts off communication between the orbit roll 26 and thesteering valve 25 and allows communication between the leverelectromagnetic control valve 27 from the steering valve 25.

(Switching Valve 52)

The switching valve 52 is a valve for switching the priority controlvalve 51 between the handle stage Sh and the lever stage S1. Thehydraulic pressure from left and right pilot oil passages PL and PR isinput to the pilot port of the switching valve 52. The switching valve52 is switched to the shutoff stage Sb when a differential pressurebetween the left and right pilot oil passages PL and PR occurs, and isswitched to the communication stage St when the pressure in both oilpassages PL and PR is the same.

The left and right pilot oil passages PL1 and PR1 for the lever areconnected to the input ports of the switching valve 52 via a shuttlevalve 54. The output ports of the switching valve 52 are connected tothe pilot ports of the priority control valve 51.

(Steering Operation)

The following is an explanation of a steering device operations whenturning to the left in the same way as the first embodiment.

(Handle 21 Operation)

FIG. 6 illustrates the flow of steering operating fluid (solid lines)and pilot operating fluid (dashed lines) when the handle is operated fora turn to the left.

When the handle 21 is operated, operating fluid is input from the firstinput oil passage P01 to the orbit roll 26. The operating fluid isoutput from the L port of the orbit roll 26 but operating fluid is notoutput from the R port. The R port is connected to the tank 31 via the Tport. Therefore, while a certain pilot hydraulic pressure is produced inthe left pilot oil passage PL, the hydraulic pressure in the right pilotoil passage PR is substantially “0”. In this case, the switching valve52 is switched to the shutoff stage Sb.

When the switching valve 52 is switched to the shutoff stage Sb, pilothydraulic pressure is not supplied to the priority control valve 51. Asa result, the priority control valve 51 is switched to the handle stageSh.

In the condition described above, the operating fluid input from thefirst input oil passage P01 is output from the orbit roll 26 to the leftpilot oil passage PL. The operating fluid is input to the left pilotport of the directional control valve 25 a of the steering valve 25 viathe stop valve 35. The directional control valve 25 a is switched to theleft turning stage L due to the pilot hydraulic pressure.

At this time, the operating fluid in the right pilot oil passage PR isreturned from the tank port T of the orbit roll 26 to the tank 31 viathe drain oil passage Pd.

When the directional control valve 25 a of the steering valve 25 isswitched to the left turning stage L, operating fluid from the pump 30is supplied to the bottom-side oil chamber of the left steering cylinder23L and to the rod-side oil chamber of the right steering cylinder 23Rvia the steering valve 25.

The operating fluid inside the bottom-side oil chamber of the rightsteering cylinder 23R and inside the rod-side oil chamber of the leftsteering cylinder 23L is returned to the tank 31 via the steering valve25.

(Joystick 22 Operation)

FIG. 7 illustrates a case in which only the joystick 22 is operated.When the handle 21 is not operated, operating fluid does not flowbetween the orbit roll 26 and the priority control valve 51 in the leftand right pilot oil passages PL and PR. Therefore, no pressuredifference is produced between the pilot oil passages PL and PR. In thiscase, the switching valve 62 is switched to the communication stage St.

When the joystick 22 is operated, operating fluid is input into theswitching valve 52 via the shuttle valve 54 since the operating fluid isflowing in the left lever pilot oil passage PL1. Since the switchingvalve 52 is switched to the communication stage St, the input operatingfluid is input into the pilot ports of the priority control valve 51. Asa result, the priority control valve 51 is switched to the lever stageS1.

In the condition described above, operating fluid input from the secondinput oil passage P02 is output from the joystick 22 to the left leverpilot oil passage PL1. The operating fluid is supplied to the left pilotoil passage PL via the priority control valve 51 and, in the same way asdescribed above for the handle operation, is input to the left pilotport of the directional control valve 25 a of the steering valve 25.Therefore, the directional control valve 25 a is switched to the leftturning stage L. The operating fluid in the right pilot oil passage PRflows to the drain oil passage Pd via the priority control valve 51 andthe lever electromagnetic control valve 27 and is returned to the tank31.

The flow of the steering operating fluid is exactly the same as for thehandle operation.

(When the Handle 21 and the Joystick 22 are Operated)

The joystick 22 may be operated when the operator mistakenly touches thejoystick 22 while operating the handle 21. In this case, the handleoperation is prioritized.

Specifically, when the handle 21 is operated, a differential pressure isproduced between the left and right pilot oil passages PL and PR asdescribed above. In this case, the switching valve 52 is switched to theshutoff stage Sb as illustrated in FIG. 6. Therefore, the operatingfluid from the lever electromagnetic control valve 27 is shut off by theswitching valve 52 and pilot hydraulic pressure is not supplied to thepriority control valve 51. As a result, the priority control valve 51 isswitched to the handle stage Sh in the same way as when the handle isoperated.

As described above, a handle operation is prioritized by shutting offthe operating fluid flowing from the lever electromagnetic control valve27 with the priority control valve 51 even if the joystick 22 isoperated during a handle operation.

(Effects of Second Embodiment)

(1) Handle operations are prioritized when the handle 21 is operated.Therefore, an unintentional steering operation can be prevented even ifthe operator mistakenly touches the joystick 22 during a handleoperation.

(2) The switching valve 52 is controlled by differential pressurebetween the left and right pilot oil passages PL and PR produced by ahandle operation, and as a result the priority control valve 51 isswitched to the handle stage Sh. Therefore, operations can be performedin a stable manner with a simple circuit configuration.

Third Embodiment

FIG. 8 illustrates a steering device according to a third embodiment ofthe present invention. Only a handle prioritization valve 60 in thesteering device is different from the first and second embodiments andother configurations are the same as those in the other embodiments. Thefollowing is an explanation of the handle prioritization valve 60 thatis different from the first embodiment.

(Handle Prioritization Valve 60)

The handle prioritization valve 60 includes a priority control valve 61provided in the left and right pilot oil passages PL and PR, and aswitching valve 62 for switching the priority control valve 61.

(Priority Control Valve 61)

The left and right output ports of the orbit roll 26 are connected toinput ports of the priority control valve 61 via the left and rightpilot oil passages PL and PR. The left and right output ports of thelever electromagnetic control valve 27 are connected to other inputports of the priority control valve 61. The input ports of the prioritycontrol valve 61 and the left and right output ports of the leverelectromagnetic control valve 27 are connected to each other by the leftlever pilot oil passage PL1 and the right lever pilot oil passage PR1.The priority control valve 61 is able to switch between the handle stageSh and the lever stage S1 based on pilot hydraulic pressure from theswitching valve 62. Specifically, the priority control valve is switchedto the handle stage Sh when pilot hydraulic pressure is not supplied,and switched to the lever stage S1 when pilot hydraulic pressure issupplied from the switching valve 52.

The handle stage Sh allows communication between the orbit roll 26 andthe steering valve 25 and shuts off communication between the leverelectromagnetic control valve 27 and the steering valve 25. The handlestage S1 shuts off communication between the orbit roll 26 and thesteering valve 25 and allows communication between the leverelectromagnetic control valve 27 from the steering valve 25.

(Switching Valve 62)

The switching valve 62 is a valve for switching the priority controlvalve 61 between the handle stage Sh and the lever stage S1. The leftand right pilot oil passages PL and PR are connected to the pilot portsof the switching valve 62 via a shuttle valve 63. The left and rightlever pilot oil passages PL1 and PR1 are connected to the input ports ofthe switching valve 62 via a shuttle valve 64. The output ports of theswitching valve 62 are connected to the pilot ports of the prioritycontrol valve 61.

When operating fluid flows in either of the left and right pilot oilpassages PL and PR, pilot hydraulic pressure is input into the pilotports of the switching valve 62 via the shuttle valve 63. In this case,the switching valve 62 is switched to the shutoff stage Sb. Whenoperating fluid is not flowing in the left and right pilot oil passagesPL and PR, the switching valve 62 is switched to the communication stageSt.

(Steering Operation)

The following is an explanation of a steering device operation whenturning to the left in the same way as the first embodiment.

(Handle 21 Operation)

FIG. 9 illustrates the flow of steering operating fluid (solid lines)and pilot operating fluid (dashed lines) when the handle is operated fora turn to the left.

When the handle 21 is operated, operating fluid is input from the firstinput oil passage P01 to the orbit roll 26. The operating fluid is inputinto the pilot ports of the switching valve 62 via the shuttle valve 63.Therefore, the switching valve 62 is switched to the shutoff stage Sb.When the switching valve 62 is switched to the shutoff stage Sb, pilothydraulic pressure is not supplied to the priority control valve 61.Therefore, the priority control valve 61 is switched to the handle stageSh.

In the condition described above, the operating fluid input from thefirst input oil passage P01 is output from the orbit roll 26 to the leftpilot oil passage PL. The operating fluid is input to the left pilotport of the directional control valve 25 a of the steering valve 25 viathe stop valve 35. The directional control valve 25 a is switched to theleft turning stage L due to the pilot hydraulic pressure.

At this time, the operating fluid in the right pilot oil passage PR isreturned from the tank port T of the orbit roll 26 to the tank 31 viathe drain oil passage Pd.

When the directional control valve 25 a of the steering valve 25 isswitched to the left turning stage L, operating fluid from the pump 30is supplied to the bottom-side oil chamber of the left steering cylinder23L and to the rod-side oil chamber of the right steering cylinder 23Rvia the steering valve 25.

The operating fluid inside the bottom-side oil chamber of the rightsteering cylinder 23R and inside the rod-side oil chamber of the leftsteering cylinder 23L is returned to the tank 31 via the steering valve25.

(Joystick 22 Operation)

FIG. 10 illustrates a case in which only the joystick 22 is operated.When the handle 21 is not operated, operating fluid does not flowbetween the orbit roll 26 and the priority control valve 61 in the leftand right pilot oil passages PL and PR. Therefore, pilot hydraulicpressure is not supplied to the pilot ports of the switching valve 62.In this case, the switching valve 62 is switched to the communicationstage St.

When the joystick 22 is operated, operating fluid is input into theswitching valve 62 via the shuttle valve 64 since the operating fluid isflowing in the left lever pilot oil passage PL1. Since the switchingvalve 62 is switched to the communication stage St, the input operatingfluid is input into the pilot ports of the priority control valve 61. Asa result, the priority control valve 61 is switched to the lever stageS1.

In the condition described above, operating fluid input from the secondinput oil passage P02 is output from the joystick 22 to the left leverpilot oil passage PL1. The operating fluid is supplied to the left pilotoil passage PL via the priority control valve 61 and, in the same way asdescribed above for the handle operation, is input to the left pilotport of the directional control valve 25 a of the steering valve 25.Therefore, the directional control valve 25 a is switched to the leftturning stage L. The pilot oil in the right pilot oil passage PR isreturned from the tank port T of the lever electromagnetic control valve27 to the tank 31 via the drain oil passage Pd.

The flow of the steering operating fluid is exactly the same as for thehandle operation.

(When the Handle 21 and the Joystick 22 are Operated)

The joystick 22 may be operated when the operator mistakenly touches thejoystick 22 while operating the handle 21. In this case, the handleoperation is prioritized.

Specifically, when the handle 21 is operated, operating fluid flows inthe left and right pilot oil passages PL and PR as described above. Inthis case, the switching valve 62 is switched to the shutoff stage Sb asillustrated in FIG. 9. Therefore, operating fluid from the leverelectromagnetic control valve 27 is shut off by the switching valve 62and pilot hydraulic pressure is not supplied to the priority controlvalve 61. As a result, the priority control valve 61 is switched to thehandle stage Sh in the same way as when the handle is operated.

As described above, a handle operation is prioritized by shutting offthe operating fluid flowing from the lever electromagnetic control valve27 with the priority control valve 61 even if the joystick 22 isoperated during a handle operation.

(Effects of Third Embodiment)

The effects of the third embodiment are the same as those of the secondembodiment.

Fourth Embodiment

FIG. 11 illustrates a steering device according to a fourth embodimentof the present invention. Only a handle prioritization valve 70 in thesteering device is different from the first to third embodiments andother configurations are the same as those in the other embodiments. Thefollowing is an explanation of the handle prioritization valve 70 thatis different from that of the first embodiment.

(Handle Prioritization Valve 70)

The handle prioritization valve 70 includes a prioritization valve body71, a throttle 72, and a check valve 73.

(Prioritization Valve Body 71)

The prioritization valve body 71 is provided in the left and right leverpilot oil passages PL1 and PR1 on the output side of the leverelectromagnetic control valve 27. More specifically, the left leverpilot oil passage PL1 and the right lever pilot oil passage PR1 areconnected to the output ports of the lever electromagnetic control valve27. The left lever pilot oil passage PL1 is connected to the left pilotoil passage PL, and the right lever pilot oil passage PR1 is connectedto the right pilot oil passage PR. The prioritization valve body 71 isprovided in the left and right lever pilot oil passages PL1 and PR1.

The prioritization valve body 71 is controlled by pilot hydraulicpressure and is switched between the communication stage St and theshutoff stage Sb. The output ports of the lever electromagnetic controlvalve 27 are connected to the left and right pilot oil passages PL andPR via the left and right lever pilot oil passages PL1 and PR1 in thecommunication stage St. The left and right pilot oil passages PL and PRare shut off from the output ports of the lever electromagnetic controlvalve 27 in the shutoff stage Sb.

(Throttle 72)

The throttle 72 is provided in the drain oil passage Pd. The drain oilpassage Pd is an oil passage that connects the T ports for draining oneach valve and the tank 31 as described above. The throttle 72 isprovided in the drain oil passage Pd between the T port for draining theorbit roll 26 and the T port for draining the lever electromagneticcontrol valve 27.

The oil passages on either side of the throttle 72 are connected to thepilot ports of the prioritization valve body 71. When the hydraulicpressure on the orbit roll 26 side of the throttle 72 is high, theprioritization valve body 71 is switched to the shutoff stage Sb.

(Check Valve 73)

The check valve 73 is provided parallel to the throttle 72. The checkvalve 73 opens when a differential pressure on either side of thethrottle 72 is equal to or greater than a certain pressure and guidesthe upstream operating fluid to the tank 31.

(Steering Operation)

The following is an explanation of a steering device operation whenturning to the left in the same way as the first embodiment.

(Handle 21 Operation)

FIG. 12 illustrates the flow of steering operating fluid (solid lines)and pilot operating fluid (dashed lines) when the handle is operated fora turn to the left.

When the handle 21 is operated, operating fluid is input from the firstinput oil passage P01 to the orbit roll 26. The operating fluid is inputto the left pilot port of the directional control valve 25 a of thesteering valve 25 via the stop valve 35. The directional control valve25 a is switched to the left turning stage L due to the input of pilothydraulic pressure.

At this time, the operating fluid in the right pilot oil passage PR isreturned from the T port of the orbit roll 26 to the tank 31 via thedrain oil passage Pd.

When the directional control valve 25 a of the steering valve 25 isswitched to the left turning stage L, operating fluid from the pump 30is supplied to the bottom-side oil chamber of the left steering cylinder23L and to the rod-side oil chamber of the right steering cylinder 23Rvia the steering valve 25.

The operating fluid inside the bottom-side oil chamber of the rightsteering cylinder 23R and inside the rod-side oil chamber of the leftsteering cylinder 23L is returned to the tank 31 via the steering valve25.

As described above, operating fluid from the T port of the orbit roll 26is returned to the tank 31 when the handle 21 is operated. The operatingfluid passes through the throttle 72. As a result, the upstream (orbitroll 26 side) pressure of the throttle 72 is higher than the downstream(lever electromagnetic control valve 27 side) pressure. Therefore, theprioritization valve body 71 is switched to the shutoff stage Sb.

(Joystick 22 Operation)

FIG. 13 illustrates a case in which only the joystick 22 is operated.When the handle 21 is not operated, operating fluid does not flow to theorbit roll 26. As a result, pilot hydraulic pressure is not supplied tothe prioritization valve body 71. Therefore, the prioritization valvebody 71 is switched to the communication stage St.

In the condition described above, operating fluid input from the secondinput oil passage P02 is output from the joystick 22 to the left leverpilot oil passage PL1. The operating fluid is supplied to the left pilotoil passage PL via the prioritization valve body 71 and, in the same wayas described above for the handle operation, is input to the left pilotport of the directional control valve 25 a of the steering valve 25.Therefore, the directional control valve 25 a is switched to the leftturning stage L. The pilot oil in the right pilot oil passage PR isreturned from the drain T port of the lever electromagnetic controlvalve 27 to the tank 31 via the drain oil passage Pd.

The flow of the steering operating fluid is exactly the same as for thehandle operation.

(When the Handle 21 and the Joystick 22 are Operated)

The joystick 22 may be operated when the operator mistakenly touches thejoystick 22 while operating the handle 21. In this case, the handleoperation is prioritized.

Specifically, when the handle 21 is operated, the operating fluid flowsfrom the orbit roll 26 to the tank 31 and the pressure on the orbit roll26 side of the throttle 72 becomes higher. As a result, pilot hydraulicpressure is supplied to the prioritization valve body 71 and theprioritization valve body 71 is switched to the shutoff stage Sb.

As described above, a handle operation is prioritized by shutting offthe operating fluid flowing from the lever electromagnetic control valve27 with the priority control valve 71 even if the joystick 22 isoperated during a handle operation.

When the flow rate inside the drain oil passage Pd becomes large, thedifferential pressure on either side of the throttle 72 becomes largerthan necessary and, as a result, high back pressure is developed in thedrain passages. At this time, the check valve 73 is opened to preventthe occurrence of problems inside the circuit due to a large backpressure. Even if the throttle 72 becomes clogged, the check valve 73 isopened when the hydraulic pressure in the drain oil passage Pd is equalto or greater than a certain pressure. As a result, the orbit roll 26 isable to function as normal and the handle operation is enabled.

(Effects of Fourth Embodiment)

A handle operation is prioritized when the handle 21 is operated and anunintentional steering operation can be prevented in the same way asdescribed above in the fourth embodiment. Actions are performed in astable manner and the circuit configuration is simple.

Other Embodiments

The present invention is not limited to the above embodiments andvarious changes and modifications may be made without departing from thespirit of the invention.

INDUSTRIAL APPLICABILITY

According to the present invention, a mechanism for prioritizing ahandle operation is achieved with a simple configuration and theactuation of the mechanism in a stable manner is realized in a wheelloader in which steering can be operated by a handle operation and alever operation.

What is claimed is:
 1. A steering device for a wheel loader in which afront frame and a rear frame are pivotably coupled in a horizontaldirection, the steering device comprising: a handle operated by rotationfor steering; a lever operated by tilting for steering; a steeringcylinder configured to pivot the front frame in relation to the rearframe by expansion and contraction; a steering valve configured tocontrol a supply direction of operating fluid to the steering cylinderwith pilot hydraulic pressure; a pump configured to supply operatingfluid to the steering cylinder via the steering valve; a handle controlvalve configured to supply pilot hydraulic pressure in order to controlthe supply direction to the steering valve according to a rotationoperation of the handle; a lever electromagnetic control valveconfigured to supply pilot hydraulic pressure in order to control thesupply direction to the steering valve according to a tilting operationof the lever; a handle prioritization valve configured to shut off thepilot hydraulic pressure supplied from the lever electromagnetic controlvalve to the steering valve according to a differential pressure or anabsolute pressure in a hydraulic circuit produced due to operating fluidflowing through the handle control valve when the handle is operated; aleft pilot oil passage provided between the handle control valve and thesteering valve; and a right pilot oil passage provided between thehandle control valve and the steering valve; the handle prioritizationvalve including: a priority control valve disposed between the handlecontrol valve and the lever electromagnetic control valve and thesteering valve, the priority control valve including a handle stage thatallows communication between the handle control valve and the steeringvalve and that shuts off communication between the lever electromagneticcontrol valve and the steering valve, and a lever stage that shuts offcommunication between the handle control valve and the steering valveand allows communication between the lever electromagnetic control valveand the steering valve; and a switching valve that switches the prioritycontrol valve to the handle stage when a differential pressure isproduced between the left pilot oil passage and the right pilot oilpassage.
 2. The steering device according to claim 1, furthercomprising: a left lever pilot oil passage provided between the leverelectromagnetic control valve and the priority control valve; and aright lever pilot oil passage provided between the lever electromagneticcontrol valve and the priority control valve, the switching valveswitching the priority control valve to the lever stage when nodifferential pressure exists between the left pilot oil passage and theright pilot oil passage and when a differential pressure is producedbetween the left lever pilot oil passage and the right lever pilot oilpassage.
 3. The steering device according to claim 2, wherein the leftand right lever pilot oil passages are connected between input ports ofthe priority control valve and output ports of the lever electromagneticcontrol valve.
 4. The steering device according to claim 3, wherein theleft and right lever pilot oil passages are connected to an input portof the switching valve via a shuttle valve.